Problem: It takes 21 minutes for 8 people to paint 8 walls. How many minutes does it take 20 people to paint 20 walls?
Explanation: Imagine that each person is assigned one wall, and all 8 people begin painting at the same time. Since everyone will finish painting their assigned wall after 21 minutes, it takes one person 21 minutes to paint one wall. If we have 20 people and 20 walls, we can again assign one wall to each person. Everyone will take 21 minutes to paint their assigned wall. In other words, it takes 21 minutes for 20 people to paint 20 walls.